# deduction guides for `std::unordered_multimap`

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< cpp‎ | container‎ | unordered multimap

Defined in header `<unordered_map>` |  |
template< class InputIt, class Hash = std::hash<iter_key_t<InputIt>>, class Pred = std::equal_to<iter_key_t<InputIt>>, class Alloc = std::allocator<iter_to_alloc_t<InputIt>> > unordered_multimap( InputIt, InputIt, typename /*see below*/::size_type = /*see below*/, Hash = Hash(), Pred = Pred(), Alloc = Alloc() ) -> unordered_multimap<iter_key_t<InputIt>, iter_val_t<InputIt>, Hash, Pred, Alloc>; | (1) | (since C++17)
template< class Key, class T, class Hash = std::hash<Key>, class Pred = std::equal_to<Key>, class Alloc = std::allocator<std::pair<const Key, T>> > unordered_multimap( std::initializer_list<std::pair<Key, T>>, typename /*see below*/::size_type = /*see below*/, Hash = Hash(), Pred = Pred(), Alloc = Alloc() ) -> unordered_multimap<Key, T, Hash, Pred, Alloc>; | (2) | (since C++17)
template< class InputIt, class Alloc > unordered_multimap( InputIt, InputIt, typename /*see below*/::size_type, Alloc ) -> unordered_multimap<iter_key_t<InputIt>, iter_val_t<InputIt>, std::hash<iter_key_t<InputIt>>, std::equal_to<iter_key_t<InputIt>>, Alloc>; | (3) | (since C++17)
template< class InputIt, class Alloc > unordered_multimap( InputIt, InputIt, Alloc ) -> unordered_multimap<iter_key_t<InputIt>, iter_val_t<InputIt>, std::hash<iter_key_t<InputIt>>, std::equal_to<iter_key_t<InputIt>>, Alloc>; | (4) | (since C++17)
template< class InputIt, class Hash, class Alloc > unordered_multimap( InputIt, InputIt, typename /*see below*/::size_type, Hash, Alloc ) -> unordered_multimap<iter_key_t<InputIt>, iter_val_t<InputIt>, Hash, std::equal_to<iter_key_t<InputIt>>, Alloc>; | (5) | (since C++17)
template< class Key, class T, typename Alloc > unordered_multimap( std::initializer_list<std::pair<Key, T>>, typename /*see below*/::size_type, Alloc ) -> unordered_multimap<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>; | (6) | (since C++17)
template< class Key, class T, typename Alloc > unordered_multimap( std::initializer_list<std::pair<Key, T>>, Alloc ) -> unordered_multimap<Key, T, std::hash<Key>, std::equal_to<Key>, Alloc>; | (7) | (since C++17)
template< class Key, class T, class Hash, class Alloc > unordered_multimap( std::initializer_list<std::pair<Key, T>>, typename /*see below*/::size_type, Hash, Alloc ) -> unordered_multimap<Key, T, Hash, std::equal_to<Key>, Alloc>; | (8) | (since C++17)
template< ranges::input_range R, class Hash = std::hash<range_key_t<R>>, class Pred = std::equal_to<range_key_t<R>>, class Alloc = std::allocator<range_to_alloc_t<R>> > unordered_multimap( std::from_range_t, R&&, typename /* see description */::size_type = /* see description */, Hash = Hash(), Pred = Pred(), Alloc = Alloc() ) -> unordered_multimap<range_key_t<R>, range_mapped_t<R>, Hash, Pred, Alloc>; | (9) | (since C++23)
template< ranges::input_range R, class Alloc > unordered_multimap( std::from_range_t, R&&, typename /* see description */::size_type, Alloc ) -> unordered_multimap<range_key_t<R>, range_mapped_t<R>, std::hash<range_key_t<R>>, std::equal_to<range_key_t<R>>, Alloc>; | (10) | (since C++23)
template< ranges::input_range R, class Alloc > unordered_multimap( std::from_range_t, R&&, Alloc ) -> unordered_multimap<range_key_t<R>, range_mapped_t<R>, std::hash<range_key_t<R>>, std::equal_to<range_key_t<R>>, Alloc>; | (11) | (since C++23)
template< ranges::input_range R, class Hash, class Alloc > unordered_multimap( std::from_range_t, R&&, typename /* see description */::size_type, Hash, Alloc ) -> unordered_multimap<range_key_t<R>, range_mapped_t<R>, Hash, std::equal_to<range_key_t<R>>, Alloc>; | (12) | (since C++23)
Exposition-only helper type aliases |  |
template< class InputIter > using iter_val_t = typename std::iterator_traits<InputIter>::value_type; |  | (exposition only*)
template< class InputIter > using iter_key_t = std::remove_const_t< std::tuple_element_t<0, iter_val_t<InputIter>>>; |  | (exposition only*)
template< class InputIter > using iter_mapped_t = std::tuple_element_t<1, iter_val_t<InputIter>>; |  | (exposition only*)
template< class InputIter > using iter_to_alloc_t = std::pair<std::add_const_t<tuple_element_t<0, iter_val_t<InputIter>>>, std::tuple_element_t<1, iter_val_t<InputIter>>>; |  | (exposition only*)
template< ranges::input_range Range > using range_key_t = std::remove_const_t<typename ranges::range_value_t<Range>::first_type>; |  | (since C++23) (exposition only*)
template< ranges::input_range Range > using range_mapped_t = typename ranges::range_value_t<Range>::second_type; |  | (since C++23) (exposition only*)
template< ranges::input_range Range > using range_to_alloc_t = std::pair<std::add_const_t<typename ranges::range_value_t<Range>::first_type>, typename ranges::range_value_t<Range>::second_type>; |  | (since C++23) (exposition only*)

1-8) These deduction guide are provided for `unordered_multimap` to allow deduction from an iterator range (overloads (1,3-5)) and std::initializer_list (overloads (2,6-8)).
9-12) These deduction guides are provided for `unordered_multimap` to allow deduction from a `std::from_range_t` tag and an `input_range`.

These overloads participate in overload resolution only if `InputIt` satisfies LegacyInputIterator, `Alloc` satisfies Allocator, neither `Hash` nor `Pred` satisfy Allocator, and `Hash` is not an integral type.

Note: the extent to which the library determines that a type does not satisfy LegacyInputIterator is unspecified, except that as a minimum integral types do not qualify as input iterators. Likewise, the extent to which it determines that a type does not satisfy Allocator is unspecified, except that as a minimum the member type `Alloc::value_type` must exist and the expression std::declval<Alloc&>().allocate(std::size_t{}) must be well-formed when treated as an unevaluated operand.

The size_type parameter type in these guides in an refers to the size_type member type of the type deduced by the deduction guide.

### Notes

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_containers_ranges` | `202202L` | (C++23) | Ranges-aware construction and insertion; overloads (9-12)

### Example

Run this code

    #include <unordered_map>

    int main()
    {
        // std::unordered_multimap m1 = {{"foo", 1}, {"bar", 2}};
            // Error: braced-init-list has no type cannot
            // deduce pair<Key, T> from {"foo", 1} or {"bar", 2}

        std::unordered_multimap m1 = {std::pair{"foo", 2}, {"bar", 3}}; // guide #2
        std::unordered_multimap m2(m1.begin(), m1.end()); // guide #1
    }

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3025 | C++17 | initializer-list guides take std::pair<const Key, T> | use std::pair<Key, T>

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